1. Field of the invention
The present invention relates to a fuel cell power system and, more particularly, a fuel cell power system provided with a fuel conditioning apparatus.
2. Description of the Prior Art
In a fuel cell power system of an indirect type, hydrocarbon fuels are converted to hydrogen-rich gas by a fuel conditioning apparatus or a fuel processer, and then supplied to the fuel cell stack in which energy of electrochemical reaction of fuel gas with oxygen from air is converted into electric power. Since the electric power generated by the fuel cell system depends on a feed rate of the fuel, an amount of the fuel gas supplied to the fuel cell stack must be increased or decreased with variation of the load. It is, however, impossible with the fuel conditioning apparatus to rapidly increase an amount of the generation of fuel gas since the rate of reforming reaction can not be increased rapidly even if an amount of the hydrocarbon fuels fed to the fuel conditioning apparatus is rapidly increased. Accordingly, if and when the load is rapidly increased during light-load running, the feed rate of the fuel gas becomes temporarily short.
In order to overcome this shortcoming, it has been proposed to provide a reservoir tank between the fuel conditioning apparatus and the fuel cell stack. In this system, the hydrogen-rich gas is reserved in the tank to supply a deficient amount of the fuel gas to the fuel cell stack when the load is rapidly increased. However, this system requires provision of a reservoir tank with a large capacity, resulting in excessive size of the system. It has also been proposed to use a fuel conditioning apparatus with a large ability to generate a larger amount of hydrogen-rich gas than the amount required for the operation of the fuel cell stack, and an excess amount of the fuel gas is returned to the fuel conditioning apparatus through a bypass provided with a flow control valve. However, this system causes considerable increase in size of the fuel conditioning apparatus and deterioration of the overall efficiency of the fuel cell power system.
Since the fuel cell reaction is accompanied by generation of heat, some part of the heat of the reaction must be disposed of in order to maintain the working temperature of the fuel cell stack at a predetermined value. For this reason, air is generally used as a cooling medium and fed to the fuel cell stack. However, such a fuel cell system requires use of a large-sized air blower and a large-sized heat exchanger because of low heat conductivity of the air (generally, 2.6.times.10.sup.-4 W/m-deg), resulting in decrease of the overall efficiency of the fuel cell system.
A phosphoric acid electrolyte fuel cell system is generally operated at a temperature ranging from 180.degree. to 200.degree. C., but the system, when being out of operations, is maintained at a temperature of about 70.degree. C. to prevent the electrolyte from deterioration. Thus, when starting up the the fuel cell system, it is required to heat the fuel cell stack to its working temperature. For this purpose, the process air and/or cooling air in the circulating line is heated by applying an electric power to an electric heater provided on the air-circulating line, or by supplying steam to the heat exchanger arranged in the circulating line. However, the use of electric heaters requires electric power and the use of steam requires an additional steam generating apparatus separate from the steam generating apparatus for supplying steam to the fuel conditioning apparatus.
It is therefore an object of the present invention to overcome the aforesaid disadvantages of the conventional fuel cell power system and to provide an improved fuel cell power system with excellent characteristics in response to load variations and in overall efficiency.
Another object of the present invention is to provide an improved fuel cell power system which makes it possible to use small-sized equipments such as an air blower, without no decrease in performance of the system.
Still another object of the present invention is to provide an improved fuel cell power system that make it possible to start up a fuel cell stack without use of any independent heat generating apparatus.